In an extraordinary portion of a television interview on foreign policy challenges, Tillerson was asked about claims that Trump has a short attention span, regularly repeats himself and refuses to read briefing notes.

World Bulletin / News Desk

Washington's chief diplomat Rex Tillerson found himself obliged to defend President Donald Trump's fitness for office Friday after a bombshell new book called into doubt his mental health.

"I've never questioned his mental fitness. I've had no reason to question his mental fitness," said Tillerson, whose office was last year forced to deny reports that he had referred to Trump as a "moron" after a national security meeting.

And, even in defending Trump, the former ExxonMobil chief executive admitted he has had to learn how to relay information to a president with a very different decision-making style.

"I have to learn how he takes information in, processes it and makes decisions," Tillerson told CNN. "I'm here to serve his presidency. So I've had to spend a lot of time understanding how to best communicate with him."

But Tillerson emphasized the right decisions had been made and that the United States is in a stronger place internationally thanks to Trump's policies.